FOOTING THE BILLBOARD: The Nets fired the first salvo in their free-agent battle with the Knicks, slapping this billboard right across the street from Madison Square Garden, and sparking a tantrum in Knicks owner James Dolan. Photo: Allison Joyce

The Knicks have never been threatened the way they are now by the Nets. Julius Erving led the 1976 Nets to a championship, but that was on Long Island, in the ABA. The Jason Kidd Nets who reached the Finals were little more than fleeting nuisance.

This is different.

This is war — a sudden Cold War between a charismatic billionaire Russian owner named Mikhail Prokhorov, who vows to make beautiful music with dynamic hip hop mogul Jay-Z, and James Dolan, whose worst nightmare is no longer Isiah Thomas, but the growing possibility that LeBron James will be the third face on that towering, swaggerlicious “blueprint for greatness” billboard taunting the Garden and the Knicks on 34th and Eighth.

Russian woo-lette versus the Knicks’ desperate rush to lock up Amar’e Stoudemire so this historic, franchise-changing game of musical free agents does not leave Dolan and team president Donnie Walsh standing without a chair to sit in.

If Dolan was apoplectic enough to telephone Jay-Z and the league to complain about the billboard, imagine his reaction would be to this Marv Albert call at the Garden: “Brook Lopez throws it back to LeBron . . . LeBron from the corner for the win . . . Yes!” On YES, no less. That would make Dolan as cheery as George Steinbrenner whenever he lost the Mayor’s Trophy game to the Mets.

So now the Knicks privately have accused the Nets of a behind-the-scenes smear campaign that reluctantly has forced them to veer off the high road and get down and dirty over the matter of which side truly gave the better presentation to King James on Thursday in Cleveland.

You see, at stake, for the first time, is only the basketball soul of this great city — where the City Game has belonged to the Knicks since 1946.

This is the basketball version of the NFL finding itself under siege from the AFL, except in this instance, there will be no Knicks-Nets merger.

It’s no wonder the Knicks are feeling the heat.

The new kids on the block — go-getters Prokhorov, Jay-Z and head coach Avery Johnson — are making the Knicks sweat bullets. Both before and after each team’s pitch.

“In this situation,” Johnson said yesterday, “it’s either home run or strikeout. One team’s gonna hit a home run. Everybody else is gonna strike out.”

There are varying degrees of LeBron-or-bust. If he decides to join the Bulls, or stays home in Cleveland, the Knicks will have struck out, no matter what else they manage to cobble together. But if The King brings his magic and his majesty to the Nets? There won’t be a Knicks executive — or fan — who won’t feel like Bill Buckner.

Johnson is optimistic of his team’s chances of landing James.

“1 to 10? After what I saw, I would hope it’s an . . . 8,” Johnson said. “I would hope so. But who knows?”

James with the Nets would make the Knicks more irrelevant than ever, and it doesn’t matter if he’s playing in Newark (is that Spike Lee at the Prudential Center, the World’s Most Famous Arena?), or Brooklyn, or Moscow.

As the shot clock winds down, it is up to Walsh, more than anyone, to avoid the unthinkable.

Dolan brought him here for this moment. Walsh brought coach Mike D’Antoni here for this moment. The Nets don’t really have a team president, with Rod Thorn leaving, and it doesn’t seem to matter. Walsh has had two years to figure this out. We find out now how good he is.

He needs to build a team, and fast. If he builds it, if he can somehow trade for James buddy Chris Paul, maybe LeBron will come after all, because nothing is etched in stone here.

The Knicks read that James might have been taken aback by Walsh, because of his recent neck surgery, showing up in a wheelchair, and finger the Nets as subversives. That the Knicks’ billion-dollar pitch to James didn’t exactly have him leaping to his feet panting, “Where do I sign?”

The Knicks’ brass-knuckled response? Ha! The Nets’ presentation wasn’t all that, and who says they have a better shot than we do?